Satellite telephone systems for fixed and mobile communications are emerging as a new global business. These systems utilize many individual circuits routed through one satellite or a constellation of many satellites to effect communications. The value of the satellite telephone system is that it provides ubiquitous coverage of large areas of the earth without the construction of many small terrestrial cells. Since the allocation of frequencies for satellite services, a number of proposals have been advanced for the deployment of satellite communications systems. In general, these proposals have involved either a Time Division Multiple Access (TDMA) technique or a Code Division Multiple Access (CDMA) technique.
In addition, there have been two general proposals advanced for the satellite operation itself. The first proposal is for an onboard processing design which involves processing and multiplexing, for feederlink bandwidth reduction, on the satellite itself. Onboard processing involves reducing an uplinked communications signal to baseband (i.e., to digital bits), and then possibly switching the signal, via inter-satellite links, to another satellite for downlinking. The second proposal uses a "bent" pipe satellite transponder as a classical repeater to receive, frequency shift, and transmit (repeat) signals without any processing on the satellite or any reduction of the signals to baseband.
With the first type of system (i.e., the onboard processing system) a terrestrial gateway, which functions as a ground insertion point to the Public Switched Telephone Network (PSTN), may be located at any arbitrary place. Onboard processing of signals has many advantages which can be traded off against the simplicity of the "bent pipe" repeater. One significant advantage of onboard processing is that the user communications traffic signals (e.g., voice and/or data), and any required signaling for control of user terminals and other devices, is established and performed o n the satellite. Furthermore, as in CDMA systems, self-interference can be avoided on the down links, thereby increasing the capacity of the system. Further it can be appreciated that routing of signals between various satellite node points can be effected, through inter-satellite links, thereby allowing a significant amount of flexibility in call connection. Finally, conservation of spectrum can be effected by utilizing that portion of the spectrum which may be unused due to the system inefficiency of the "bent pipe" architecture.
It should thus be appreciated that an advantage of on-board processing is that signals arriving at or being sent from the satellite may be controlled on the satellite by information received at the satellite, or as relayed from the satellite to a control point. Typically this control point has been a ground station.
It should thus be further appreciated that this latter technique requires that the control point (e.g., ground station) must be made more complex in order to participate in the control loop. It would be desirable, then, to simplify the overall system and control point complexity.
Reference in this regard can be had to the following U.S. Pat. Nos.: 4,991,199, Saam, "Uplink Power Control Mechanism For Maintaining Constant Output Power From Satellite Transponder"; 4,752,967, Bustamante et al., "Power Control System For Satellite Communications"; 5,339,330, Mallinckrodt, "Integrated Cellular Communications System"; 4,752,925, Thompson et al., "Two-Hop Collocated Satellite Communications System"; 5,126,748, Ames et al., "Dual Satellite Navigation System And Method"; 5,109,390, Gilhousen et al., "Diversity Receiver In A CDMA Cellular Telephone System"; and 5,138,631, Taylor, "Satellite Communication Network".
Reference can also be had to commonly assigned and allowed U.S. patent application: Ser. No.: 08/467,209, filing date: Jun. 06, 1995, entitled "Closed Loop Power Control For Low Earth Orbit Satellite Communications System", by Robert A. Wiedeman and Michael J. Sites.